The present invention relates to a video signal correction system for correcting the saturation of a color signal to an optimum level in accordance with the amount of correction of the luminance signal in the process of tone correction (black level correction, gamma correction, etc.) of the video luminance signal for such appliances as television receivers, video tape recorders, video cameras and video discs.
With the increase in size and improvement in quality of the color television receiver in recent years, the video signal correction system has been used for enlarging the dynamic range of the image on the cathode ray tube (hereinafter called "CRT") by tone correction of the video luminance signal through a nonlinear amplifier in order to produce a clearer image.
A conventional video signal correction system will be explained. FIG. 6 is a block diagram showing a conventional video signal correction system. In FIG. 6, reference numeral 1 designates a luminance signal correction circuit for producing a corrected luminance signal by correcting the tone of the luminance signal input. This luminance signal correction circuit includes, for example, a black level correction circuit or a gamma correction circuit. Numeral 2 designates a contrast-brightness control circuit for altering the direct-current (hereinafter called "DC") bias level and contrast of the corrected luminance signal, numeral 3 a matrix circuit for producting a color signal from the luminance signal and a color difference signal, and numeral 4 a CRT. Numeral 5 designates a color demodulation circuit for demodulating the color carrier signal and producing the color difference signal, numeral 9 an adder for adding the contrast control voltage and the color control voltage to each other, numeral 6 a delay circuit for matching the time differences of the corrected luminance signal inputted to a divider circuit 7 and the delay luminance signal outputted from the delay circuit 6, numeral 7 a divider circuit for calculating the amount of correction of the luminance signal by dividing the corrected luminance signal by the delayed luminance signal and producing a color amplitude correction signal, and numeral 8 a multiplier circuit for subjecting the amplitude of the input color carrier signal to gain control by the color amplitude correction signal and producing a corrected color carrier signal. The circuits other than the CRT 4 may be configured of either analog or digital circuits or a combination thereof.
The operation of the video signal correction circuit configured as described above will be explained below. FIG. 7 shows waveforms produced at various parts of the circuit.
First, a luminance signal a inputted to the circuit is applied to the luminance signal correction circuit 1 and the tone of the luminance signal is corrected (as black level correction, gamma correction, etc., as an example) to produce a corrected luminance signal b. Let the level of the luminance signal a be Ey. From the specification of the NTSC system, equation (1) is given by: EQU Ey=0.3Er+0.59Eg+0.11Eb (1)
where Er is the voltage of the color signal R (red), Eg the voltage of the color signal G (green), and Eb the voltage of the color signal B (blue).
Assuming that the amount of brightness correction at a point on the image is A, the corrected luminance signal b is given as A.times.Ey. This corrected luminance signal b is applied to the brightness control circuit 2, and after the amplitude and the DC bias level thereof are adjusted by the contrast control voltage g and the brightness control voltage h, an output luminance signal c is produced. In other words, assuming that the amount of gain correction by contrast control is C, the output luminance signal c is given as A.times.C.times.Ey.
The luminance signal a is delayed by the delay circuit 6 so as to be inputted to the divider circuit 7 at time as the corrected luminance signal b. The signal thus delayed is produced as the delayed luminance signal 1 and is applied to the divider circuit 7.
The divider circuit 7 divides the corrected luminance signal b by the delayed luminance signal 1 to detect the amount of correction of the luminance signal. Assuming that the amplitude of the delayed luminance signal 1 is equal to that of the luminance signal a, the amount A of correction of the luminance signal is expressed by the following equation (2): EQU A.times.Ey/Ey=A (2)
Although .alpha..times.A+.beta. is the actual result due to an operational error occurring in the divider circuit, there will be no problem if .alpha..apprxeq.1 and .beta.&lt;&lt;A.
This result is applied to the multiplier circuit 8 as a color amplitude correction signal m. The color carrier signal d is adjusted to an amplitude corresponding to the color amplitude correction signal m at the multiplier circuit 8. According to the NTSC system, the following equation (3) is given when En is the input color carrier signal: EQU En=(Er-Ey)/1.14.times.cos(2.times..pi..times.fs.times.t)+(Eb-Ey)/2.03.times . sin (2.times..pi..times.fs.times.t) (3)
Thus the corrected color carrier signal n is given as A.times.En, i.e., by the following equation (4): ##EQU1## The corrected color carrier signal n is applied to the color demodulation circuit 5 and is subjected to the tint control in accordance with the tint control voltage j and the color control in accordance with the color control voltage k as well as the color demodulation, after which the signal is produced as a color difference signal e. More specifically, the color difference signal e is the result of detection of equation (4), and takes the form of the corrected color carrier signal n multiplied by the amount of contrast correction C. EQU R-Y component: A.times.C.times.(Er-Ey) EQU B-Y component: A.times.C.times.(Eb-Ey) EQU G-Y component: A.times.C.times.(Eg-Ey) (5)
In these formulae, the calculated color control voltage k is obtained by operation of the adder 9 where the contrast control voltage g is added to the color control voltage i. In other words, the color control is effected in an operatively interlocked relation with the contrast control.
The color difference signals and the output luminance signal described above are all applied to the matrix circuit 3 for calculation. As a result, the color signal f shown in equation (6) is obtained. EQU R component: A.times.C.times.Ey+A.times.C.times.(Er-Ey) =A.times.C.times.Er EQU C component: A.times.C.times.Ey+A.times.C.times.(Eg-Ey) =A.times.C.times.Eg EQU B component: A.times.C.times.Ey+A.times.C.times.(Eb-Ey) =A.times.C.times.Eb (6)
The CRT 4 is driven by each voltage of the color signal f thus obtained thereby to display an image. (See JP-A-01344439, for instance).
In the conventional configuration mentioned above, however, the luminance signal a is applied in its direct form to the divider circuit 7 which divides the corrected luminance signal b by the delayed luminance signal 1. In the case where the level of the luminance signal a is near to the black level, i.e., zero, therefore, the quotient assumes a substantially infinite value, with the result that the correction of the color signal amplitude (saturation) becomes excessive.
Further, since the color signal is corrected regardless of the average picture level (hereinafter called "APL"), noise becomes conspicuous when a dark (low-APL) image is inputted.